Unless you are extraordinarily disciplined, chances are you eat more than you think.

It is a psychological phenomenon that has been widely researched, most prominently by Brian Wansink at Cornell University in the United States. In his book Mindless Eating, Wansink shows even people of normal weight underestimate their food intake by about 20 per cent.

People who are overweight can underestimate by more than 50 per cent. He describes the gap between what you think you eat and what you actually eat as the "mindless margin". He has done many experiments showing we can easily be fooled into eating more than we need or believe we have eaten.

There is a combination of factors at play - many to do with our eating environment - that conspire against us here.

"Portion creep" is a huge trap.

It is a term describing the relentless upsizing of portion sizes of food over the years.

You may say "I just had a muffin" or "I just had a bowl of cereal".

But research shows there can be huge variation in what a portion is.

A muffin can be bigger than your fist and pack in nearly as many calories as a meal.

A bowl of cereal could contain anything from the standard half-cup to two cups, depending on the size of your bowl and your appetite.

But in our minds, it is still one serve. Our brains focus on the number of serves, and not the amount.

Restaurant and fast-food outlets have taken advantage of this.

In the 1970s, McDonald's had one size of fries - what is now known as "small". The rationale was that if people wanted more, they could order a second portion. This did not happen, though, because people felt greedy having two serves. When the bigger portion was introduced, it became easy for people to order the bigger size, even though it may have been twice as big as the smaller serve. Large portions are now everywhere from cafes to pubs to homes.

It probably comes as no surprise that larger plates also mean we eat more. Wansink's research shows that if you choose a larger plate, you will almost certainly eat more, even when you are serving yourself, and even when you are trying not to eat more.

So what can we do? The good news is we can fight back with a little bit of mindfulness. Try simply using smaller bowls or plates for a start. You will eat less without feeling deprived, because you will still have a "full" plate. Cut muffins, slices and cakes in half, and share or save the rest for another day.

And don't be sucked into "value" deals. Buy only what you really want to eat. We may feel if we are not getting a large plate full of food, it is not good value. But we need to realise that "good value" has a cost associated with it - we just pay with our bodies, not our wallets.

Niki Bezzant is a healthy cooking expert and the editor of Healthy Food Guide magazine. Latest issue on sale now. If you have a question for Niki, email editor@ healthyfood.co.nz with SST in the subject line.